The invention relates to a method for the manufacture of a superconductor with a superconductive intermetallic compound consisting of at least two chemical elements in general and more particularly to such a method wherein a ductile structural part made of at least one element of the compound is brought into contact with a second structural part which contains a carrier metal for the remaining elements of the compound; the remaining elements of the compound are subsequently admitted to the second ductile structural part at elevated temperature; and the compound is formed by a reaction of its remaining elements, which diffuse through the second structural part, with the first structural part.
Superconductive intermetallic compounds of the type A.sub.3 B, consisting of two chemical elements, for example, Nb.sub.3 Sn or V.sub.3 Ga, which have an A-15 crystal structure exhibit very good superconductor properties being particularly distinguished by a high critical magnetic field, a high transition temperature and a high critical current density. As a result, they are particularly well suited as superconductors for superconducting coils used to generate strong magnetic fields such as are needed for research purposes. Other possible applications include superconducting magnets for the suspension guidance of magnetic suspension railroads and the windings of electric machines. Recently, ternary compounds such as niobium-aluminum-germanium (Nb.sub.3 Al.sub.0.8 Ge.sub.0.2) have also become of special interest. However, since these compounds are very brittle, their manufacture in a form suitable, for example, for use in magnet coils presents considerable difficulties. Several methods of manufacturing superconductors in the form of long wires or ribbons with, in particular, two-component intermetallic compounds have been disclosed. These methods are used particularly for the manufacture of what are known as multi-core conductors. Such conductors have wires, typically of Nb.sub. 3 Sn or V.sub.3 Ga, arranged in a normal-conducting matrix. In the known methods, a ductile chemical element, in wire form, of the compound to be prepared, e.g., a niobium or vanadium wire, is surrounded with a sleeve of an alloy containing a ductile carrier metal and the remaining elements of the compound, e.g., a copper-tin alloy or a copper-gallium alloy. In particular, a multiplicity of such wires are embedded in a matrix of the alloy. The structure so obtained is then subjected to cross-section reducing processing to obtain a long wire such as is required for coils. As a result the diameter of the wires, e.g. niobium or vanadium wire, for example, is reduced to a small value in the order of magnitude of about 30 to 50 .mu.m or even less, which is desirable in view of the superconduction properties of the conductor. Through the cross-section reducing processing attempts are also made to obtain the best possible metallurgical bond between the wire and the surrounding matrix material of the alloy without, however, the occurrence of reactions which lead to embrittlement of the conductor. After the cross-section reducing processing, the conductor, consisting of one or more wires and the surrounding matrix material, is subjected to a heat treatment such that the desired compound is formed by a reaction of the wire material, i.e., the niobium or vanadium, for instance, with the further element, contained in the surrounding matrix, of the compound, e.g., tin or gallium. In this process, the element contained in the matrix diffuses into the wire material, which consists of the other element of the compound, and reacts therewith forming a layer consisting of the desired compound. (See United Kingdom Patent Specification No. 1,280,583 and 1,335,447; U.S. Pat. No. 3,728,165).
However, these known methods are not fully satisfactory for a number of reasons. First, the diffusion process in these methods cannot be carried out so that all the gallium or tin present in the matrix is used up to form the intermetallic compound. It is therefore not possible to build up V.sub.3 Ga or Nb.sub.3 Sn layers of any desired thickness. Rather, the diffusion of gallium or tin toward the vanadium or niobium cores will terminate when the activity of the elements gallium and tin in the copper matrix is equal to their activity in the intermetallic compounds V.sub.3 Ga or Nb.sub.3 Sn produced. In other words, no further V.sub.3 Ga or Nb.sub.3 Sn will be formed when the concentration of the gallium or the tin in the copper matrix has dropped to a given value because of the inward diffusion of gallium or tin into the cores. If, for instance, gallium is diffused from a copper-gallium matrix with 18 atom percent of gallium into vanadium cores at a temperature of about 700.degree. C., the equilibrium state mentioned, at which no further formation of V.sub.3 Ga takes place, is reached when the gallium content of the matrix has dropped to about 12 atom percent. This means that only about 38% of the gallium available in the matrix is converted into V.sub.3 Ga. The thickness of the Nb.sub.3 Sn or V.sub.3 Ga layers formed in a multi-core conductor depends therefore not only on the annealing time, the annealing temperature and the composition of the copper-gallium or the copper-tin alloy, but also on the total amount of tin or gallium available for each core, i.e., on the volume of the part of the matrix available for each individual core.
However, in order to obtain a high effective critical current density, i.e., a high critical current density referred to the total conductor cross section, it is necessary that the greatest possible layer thickness of the intermetallic compound to be produced. With the known methods described above this can be achieved only by making the ratio of the matrix share to the core share of the total cross section area of the conductor such that the growth of the layer is not limited by a limited supply of gallium or tin, i.e., a core spacing as large as possible is necessary. However, in a multi-core conductor of given cross section this requirement can be met only by either drawing out the core, for a fixed number of cores, to a very thin size during the cross-section reducing process steps, or by reducing the number of cores, if the core cross section is fixed. Neither solution is very satisfactory since, on the one hand, the drawing of the cores into extremely thin filaments presents considerable difficulties and is expensive and, on the other hand, if the number of cores is reduced, the effective current density decreases and, as a rule, is only just compensated by the thicker diffusion layers which can possibly be obtained. An arbitrary increase of the core spacings, finally, is also not possible for reasons of forming technology. For instance, if a larger number of vanadium or niobium cores is to be drawn to a uniform thinness so that their cross sections remain equal, then the core spacing must not be too great.
A further difficulty with the known methods is that the matrix material containing the embedded cores, consisting of the carrier metal and the remaining elements of the compound to be produced, is relatively difficult to deform, particularly for higher concentrations of these elements. These matrix materials have, in particular, harden quickly through cross-section reducing cold working and can then be deformed further only with great difficulty. It is therefore necessary in these methods to subject the conductor structure consisting of the cores and the matrix material to intermediate anneals for recovery and recrystallization of the matrix structure which has become brittle during the cold-working even after relatively small deformation steps. Although these annealing treatments can be performed at temperatures and annealing times at which, as a rule, is below that at which the superconductive compound to be produced is formed, they are very time consuming because of their frequent repetition. This increasingly difficulty in deforming the matrix material with increasing content of the remaining elements of the compound to be produced is, finally, also a reason why the concentration of, for instance, gallium or tin cannot simply be increased arbitrarily in the matrix in order to obtain heavier layers of the compound to be produced. Furthermore, with increasing concentration of these elements, the melting point of the matrix material drops. For very high concentrations, this leads to problems in the heat treatment for forming the intermetallic compound. Furthermore, if the concentration is too high these elements can form undesirable intermetallic phases with the carrier metal.
Proposed methods are also known in which the repeated intermediate anneals mentioned are eliminated. In these methods one or more cores of a ductile element of the compound to be produced, particularly niobium or vanadium, are embedded in a ductile matrix material, e.g., copper, silver or nickel, which contains, at most, only very small amounts of the element of the compound to be produced. The structure consisting of the cores and this matrix material can then be processed without any intermediate anneal by a cross-section reducing process, e.g., by cold-drawing, into a thin wire which contains very thin cores of vanadium or niobium. After the last cross-section reducing process step, the remaining elements of the compound to be produced, e.g., tin in the case of Nb.sub.3 Sn, are then applied to the matrix material. This is done by briefly immersing the wire in a tin melt, so that a thin tin layer is formed on the matrix material, or by evaporating a tin layer on the matrix material. Subsequently, a heat treatment is performed, in which the elements of the compound to be produced, which have been applied to the matrix material, are first diffused into the matrix material and then through it, and then form the desired superconductive compound through reaction with the cores (see "Applied Physics Letters" Vol. 20 (1972), pages 443 to 445; U.S. Pat. No. 3,829,963).
However, only relatively small amounts of an element such as tin can be applied to the matrix which, for instance, consists of copper, since in applying larger amounts of tin an undesirable, brittle intermediate phase of copper and tin can readily form at the temperature necessary for diffusing the tin into the copper matrix. After excessive amounts of tin have been diffused into the matrix, the tin itself or a surface area of the matrix can melt and in the process can easily drip or run off from the matrix surface. Therefore, in this method also, only a limited amount of the lower melting temperature element, e.g., tin, is available for the formation of the desired intermetallic compound, e.g., Nb.sub.3 Sn. In U.S. Pat. No. 3,829,963, it is suggested that, if desired, all the niobium contained in the copper matrix can also be converted into Nb.sub.3 Sn, if the individual process steps for coating the matrix with tin are repeated often enough to obtain the subsequent formation and homogenization of the copper-tin matrix and for reacting the tin contained in the matrix with the niobium core. However, such a method is extremely expensive because of the large number of process steps required.
In U.S. Pat. No. 3,829,963, a continuous method for the manufacture of Nb.sub.3 Sn multi-core conductors, in which a conductor structure in wire form, consisting of a copper matrix and embedded niobium cores, is continuously conducted through an oven, in which several containers with melted tin are arranged side by side is also described. The parts of the interior of the oven located above the respective containers are traversed by the conductor structure sequentially. The first tin melt, through whose associated vapor space the conductor structure first runs, is at a temperature of 1500.degree. C., and the other tin melts, through whose vapor spaces the conductor structure runs subsequently, are at a temperature of 1000.degree. C. The conductor itself is kept at a temperature of 850.degree. C. by the oven. As described in the U.S. Pat. No. 3,829,963, the tin vapor pressure in the vapor space above the first tin melt which is at a temperature of 1500.degree. C., must be high enough so that the transfer and deposition rate of the tin exceeds the solid diffusion rate of the tin into the copper matrix to cause a tin concentration gradient to build up rapidly transversely across the wire radius. The conductor structure in wire form is kept over the tin melt of higher temperature until sufficient tin for the formation of the desired mean matrix composition is applied. As also stated in U.S. Pat. No. 3,829,963, the tin vapor pressure in the vapor spaces above the tin melts which are at a temperature of 1000.degree. C., through which the conductor structure runs subsequently, must be just large enough so that the tin supply rate is reduced to a value at which tin diffuses through the copper matrix and arrives through solid diffusion at the surface of the niobium cores. The solid diffusion itself takes place at a temperature of 850.degree. C. This temperature is chosen considerably lower than the temperature of the tin melts in order to prevent re-evaporation of the tin from the matrix and melting of the matrix. This method is also extremely expensive because of the three different temperatures required for the tin melts and the conductor structure itself, all of which must be maintained accurately during the relatively laborious process. Furthermore, the temperatures of 1500.degree. and 1000.degree. C., respectively, required for the tin melts, are uncomfortably high with regard to the stresses of the container material that occur. In addition, it is difficult to reproducibly achieve a desired, given concentration of tin in the copper matrix in the vapor space above a tin melt.